II. Indications

  1. Differentiate Proximal RTA from Distal RTA
  2. Requires that serum bicarbonate be normal

III. Calculation

  1. FE-HCO3 = (uHCO3 x sCr) / (sHCO3 x uCr)
  2. Annotation
    1. Where FE-HCO3 is Fractional Excretion of Bicarbonate
    2. Where uHCO3 is urine bicarbonate
    3. Where sHCO3 is serum bicarbonate
    4. Where uCr is Urine Creatinine
    5. Where sCr is Serum Creatinine

IV. Interpretation

  1. FE-HCO3 <5%: Distal RTA
  2. FE-HCO3 >15%: Proximal RTA
    1. Assumes serum bicarbonate >20 meq/L

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